Megawad

A megawad is a fan-made PWAD that, according to the idgames archive definition, contains 15 or more levels.

Episode replacement
An episode replacement (or simply episode) is a set of fan-made levels containing a similar amount of levels as an episode of the game the episode replacement was made for. Most episode replacements therefore include eight or nine levels, which is comparable to original Doom's episode structure of eight levels and a secret level.

History
While the ultimate origin of the word will probably never be determined, the term "mega-WAD" was used by Denis Möller of The Innocent Crew in 1995 to describe the Memento Mori project. By 1996 Yonatan Donner used the word "megawad" to describe his upcoming project, Hell Revealed. By 1997 the word was in common usage within the Doom community.

SIGIL
In December 2018, John Romero announced his Ultimate Doom PWAD episode SIGIL as a "megawad", bringing renewed attention to the term, despite being technically incorrect according to the more common Doom community interpretation, which does not count a separate deathmatch arena within a level as being a separate level.

Full game replacement

 * 2002 A Doom Odyssey
 * ConC.E.R.Ned
 * Doom the Way id Did
 * Base Ganymede: Complete
 * Wonderful Doom

Episode replacement

 * Double Impact
 * Classic Episode
 * Crusades
 * Fava Beans
 * Knee-Deep in ZDoom
 * Phobos: Anomaly Reborn
 * The Classic Episode, Part 2
 * Thy Flesh turned into a draft-excluder

Full game replacement

 * 10 Sectors
 * Alien Vendetta
 * Ancient Aliens
 * Back to Saturn X
 * Claustrophobia 1024
 * Claustrophobia 1024 2
 * Community Chest
 * Community Chest 2
 * Community Chest 3
 * Community Chest 4
 * Congestion 1024
 * Demonfear
 * Doom 2 the Way id Did
 * Eternal Doom
 * Going Down
 * GothicDM
 * Hell Revealed
 * Hell Revealed II
 * Icarus: Alien Vanguard
 * Japanese Community Project
 * Jenesis
 * Kama Sutra
 * Memento Mori
 * Memento Mori II
 * NeoDoom
 * Plutonia 2
 * Plutonia: Revisited Community Project
 * Requiem
 * Resurgence
 * Reverie
 * Revolution!
 * Scythe
 * Scythe 2
 * Speed of Doom
 * Sunlust
 * Swift Death
 * Unholy Realms
 * Valiant
 * Vile Flesh
 * Whispers of Satan
 * ZPack - Random Maps for ZDoom

Episode replacement

 * Combat Shock 2
 * Dark7
 * The Darkening
 * The Darkening Episode 2
 * Dystopia 3
 * Lunatic
 * Scythe X
 * SlayeR (WAD)
 * Stardate 20X6
 * UAC Ultra
 * Vanguard